Atomic Structure, Composition, Mechanisms and Dynamics of Transformation Interfaces in Diffusional Phase Transformations

Abstract

This overview describes progress that has been made in understanding the atomic structure, composition, mechanisms and dynamics of transformation interfaces in diffusional transformations involving plate-shaped transformation products in the past eight years. Some of the important developments in the area of structure include recognition that diffusional phase transformations obey many of the same crystallographic principles developed for martensite, widespread application of high-resolution transmission electron microscopy (HRTEM) and atomistic modelling to understand interphase boundary structure and energetics, and determination of the atomic structure and energetics of high-index ledged interfaces. In the area of composition, recent development of thermal field-emission and energy-filtering HRTEM allows compositional profiling at precipitate interfaces with subnanometer spatial resolution. In addition, the position sensitive atom-probe (PoSAP), which can determine both the mass and position of individual atoms, makes it possible to map elemental distributions in some materials with atomic resolution. These techniques are expected to provide an abundance of data on composition at transformation interfaces approaching the atomic level in the near future. In terms of the dynamics of transformation interfaces, application of in situ hot-stage HRTEM to precipitate growth and dissolution has provided information on the atomic mechanisms and kinetics of ledge motion and interaction, the mechanisms and kinetics of kink nucleation, and mechanisms of faceting/roughening of precipitate plates at the atomic level. Also, kinetic models for ledge growth which are able to treat multiple-ledge interactions and time-dependent growth have been developed and are available for comparison with experimental data. In addition to discussion of these and other major developments, effort is made to emphasize current strengths and limitations in each area and indicate possible directions for future research.